1. Field
Embodiments of the present invention generally relate to the field of imagery. More specifically, embodiments of the present invention refer to reducing flare in an optical system containing a dichroic cut filter.
2. Background
Digital camera sensors, such as CMOS and CCD sensors, are sensitive to wavelengths from approximately 380 nm to at least 1000 nm. The human eye, on the other hand, may only process colors residing in about the 400-700 nm range. As such, to process images only with colors visible to the human eye, short (e.g., <380 nm) and long (e.g., >700 nm) wavelengths of light must be filtered from the image. One technique to filter these undesirable wavelengths of light is to employ an ultra-violet (UV) cut filter to filter the shorter wavelengths of light and an infrared (IR) cut filter to filter the longer wavelengths of light. These cut filters attenuate the shorter and longer wavelengths of light, while transmitting wavelengths of light visible to the human eye.
There are two kinds of cut filters: absorptive cut filters and reflective cut filters. Absorptive cut filters are made with special dyes disposed on optical glass, whereas reflective cut filters are composed of several layers of sub-wavelength material disposed on an optical surface. A reflective cut filter may also be referred to as a dichroic cut filter. FIG. 1 illustrates a transmission curve for a UV/IR cut filter, where the filter transmits light within a passband 120 and attenuates light outside of cut bands 110 and 130. Absorptive and reflective cut filters transmit most light (e.g., >90% transmission of light) within passband 120 and block most wavelengths of light (e.g., <10% transmission of light) within cut bands 110 and 130, where cut bands 110 and 130 may span from, for example, 20-50 nm.
It is desirable to design the UV/IR cut filter with sharp cut bands such that the filter not only transmits wavelengths of light within passband 120 but also reduces the effect of “flare” on an image sensor. Flare will be discussed with respect to an image device 200 illustrated in FIG. 2. FIG. 2 illustrates an image device 200 with an optical system 202 incorporating a UV/IR cut filter 205. A digital single-lens reflex camera is an example of an image device with UV/IR cut filter 205 positioned between an image sensor 201 and optical system 202. Flare refers to light 207 that reaches image sensor 201 after reflecting off two or more surfaces in optical system 202. For example, as illustrated in FIG. 2, image light 203 may enter optical system 202 through aperture 206, pass through optical system 202, and reflect off a surface of UV/IR cut filter 205 as reflected light 207. Reflected light 207 is reflected back through UV/IR cut filter 205 by lens surface 204 prior to reaching image sensor 201. Reflected light 207 then causes a ghost image to appear on image sensor 201. Reflected light 207 may reflect off multiple surfaces in optical system 202. For example, reflected light 207 may reflect off walls in an assembly of optical system 202, dust particles in optical system 202, imperfections on optical surfaces in optical system 202, or an air/glass interface in optical system 202. Each ghost image caused by reflected light 207 may have a brightness orders of magnitude less than light from a primary image (e.g., a chief ray of light). However, in image applications with a bright source in a field of view (e.g., the sun in the background), the order of magnitude of brightness due to flare is much greater.
The effect of flare in image device 200 may be heightened by light outside of the UV/IR cut filter's passband entering optical system 202 and reaching image sensor 201. For instance, within cut bands 110 and 130, UV/IR cut filter 205 reflects approximately 50% of light and transmits 50% of light at the middle of cut bands 110 and 130. As such, a flare off UV/IR cut filter 205 may comprise one 0.5% reflection of light and one 50% reflection of light, along with a 50% transmission of light (note: most flares comprise two 0.5% reflections of light). UV/IR cut filter 205, as a result, passes more energy from flare in cut bands 110 and 130 as compared to energy from flare in passband 120. The energy from flare increases as cut bands 110 and 130 increase in width. For example, for a given passband, the energy from flare may increase approximately 50% as cut bands 110 and 130 increase from, for example, 20 to 40nm. The additional light received by image sensor 201 consequently reduces a signal/noise ratio of a processed image by image device 200 and also adds distracting artifacts to the resulting image.
As the brightness of non-image light increases, especially non-image light residing outside of the cut filter's passband, flare becomes a greater issue in the color fidelity of images.